The process or technique of producing images of an opaque object
on photographic film or on a fluorescent screen by means of radiation.
A technique of X-ray photography by which a single plane is photographed,
with the outline of structures in other planes eliminated.
(CT) is a powerful nondestructive evaluation (NDE) technique for
producing 2-D and 3-D cross-sectional images of an object from
flat X-ray images. Characteristics of the internal structure of
an object such as dimensions, shape, internal defects, and density
are readily available from CT images. Shown below is a schematic
of a CT system.
The test component is placed on a turntable stage that is between
a radiation source and an imaging system. The turntable and the
imaging system are connected to a computer so that x-ray images
collected can be correlated to the position of the test component.
The imaging system produces a 2-dimensional shadowgraph image
of the specimen just like a film radiograph. Specialized computer
software makes it possible to produce cross-sectional images of
the test component as if it was being sliced.
How a CT System Works
The imaging system provides a shadowgraph of an object, with
the 3-D structure compressed onto a 2-D plane. The density
one horizontal line of the image is uncompressed and stretched
out over an area. This information by itself is not very useful,
but when the test component is rotated and similar data for the
same linear slice is collected and overlaid, an image of the
density of the component begins to develop. To help comprehend
how this works, look at the animation below.
In the animation, a single line of density data was collected
when a component was at the starting position and then when
rotated 90 degrees. Use the pull-ring to stretch out the density
data in the vertical direction. It can be seen that the lighter
area is stretched across the whole region. This lighter area
indicate an area of less density in the component because imaging
systems typically glow brighter when they are struck with
amount of radiation. When the information from the second line
of data is stretched across and averaged with the first set
data, it becomes apparent that there is a less dense area in
the upper right quadrant of the component's cross-section.
at more angles of rotation and merged together will further
define this feature. In the movie below, a CT image of a
casting is produced.
It can be seen that the cross-section of the casting becomes
more defined as the casting is rotated, X-rayed and the stretched
information is added to the image.
In the image below left is a set of cast aluminum tensile specimens.
A radiographic image of several of these specimens is shown
CT slices through several locations of a specimen
are shown in the set of images below.
A number of slices through the object can be reconstructed to
provide a 3-D view of internal and external structural details.
As shown below, the 3-D image can then be manipulated and sliced
in various ways to provide thorough understanding of the structure.